Homogeneous organometallic catalysts are used industrially for olefin oligomerization and polymerization. For example, the Dimersol® process for dimerization of light olefins or the Alphabutol® process for selective dimerization of ethylene use homogeneous catalysts based on nickel or titanium respectively.
These organometallic catalysts generally have a high activity and excellent selectivity resulting from the unicity of the active site and the coordination sphere control of the metal.
Despite such advantages, these catalysts are likely to deactivate by interaction of the organometallic species in solution, via polynuclear species formation or dismutation mechanisms (Organometallics, 1997, 5517-5521). These interactions are favoured by the absence of repulsion between the metal centres. Besides, recycling homogeneous catalysts and/or separating them from reaction products are delicate operations in homogeneous processes.
Surface organometallic chemistry was developed to overcome these drawbacks (Angew. Chem. Inter. Ed. 2003, 42, 156-181). Homogeneous catalysts grafted on an oxide surface are recyclable and the metal centres anchored to the surface are not likely to interact with one another. However, this methodology suffers from the heterogeneity of the surface sites of a solid, which leads to a multiplicity of active sites (J. Am. Chem. Soc. 2006, 128, 9361-9370). It is furthermore difficult to control the metal content of the solid obtained or to modify the environment of the metal in order to vary the catalysis properties thereof.
A Göttingen University team has described the polymerization of ethylene catalyzed by p-oxo-heterobimetallic complexes having Al—O-M bonds, where M is a group 4 metal (Inorg. Chem. 2007, 46, 1056-1061; Inorg. Chem. 2007, 46, 7594; WO-2005/090,373). However, these complexes are neutral entities and the interactions between the catalytic centres in solution are therefore not limited by charge repulsion.
We have discovered that grafting organometallic compounds on an anion by means of at least one covalent metal-oxygen bond allows to overcome these limitations. The species formed thus has an anionic character, which affords several advantages:                the interactions between the metal centres in solution are therefore limited, as a result of the charge repulsion, and        the entity formed is soluble in ionic solvents, which opens up the possibility of immobilizing and recycling it to a two-phase technology.        